1. Field of the Invention
The present invention relates to a magnetic sensing device capable of sensing a change in a signal magnetic field at high sensitivity, method of forming the same, a magnetic sensor having the magnetic sensing device, and an ammeter having the magnetic sensing device.
2. Description of the Related Art
Generally, a magnetic recording/reproducing apparatus for writing/reading magnetic information to/from a recording medium such as a hard disk has a thin film magnetic head including a magnetic recording head and a magnetic reproducing head. The reproducing head has a giant magneto-resistive effect element (hereinbelow, GMR element) executing reproduction of a digital signal as magnetic information by using so-called giant magneto-resistive effect.
The GMR element used for a thin film magnetic head generally has a spin valve structure as shown in FIG. 19. Concretely, the GMR element is a stacked body 120 including a pinned layer 121 whose magnetization direction is pinned in a predetermined direction, a free layer 123 whose magnetization direction changes according to an external magnetic field, and an intermediate layer 122 sandwiched between the pinned layer 121 and the free layer 123 (refer to, for example, U.S. Pat. Nos. 5,159,513 and 5,206,590). Each of the top face (the face on the side opposite to the intermediate layer 122) of the pinned layer 121 and the under face (the face on the side opposite to the intermediate layer 122) of the free layer 123 is protected with a not-shown protection layer. In the pinned layer 121, specifically, as shown in FIG. 20 for example, a magnetization pinned film 124 and an antiferromagnetic film 125 are stacked in order from the side of the intermediate layer 122. The magnetization pinned film 124 may be a single layer or a synthetic layer in which a ferromagnetic layer 141, an exchange coupling film 142, and a ferromagnetic layer 143 are formed in order from the side of the intermediate layer 122 as shown in FIG. 21. The free layer 123 may be a single layer or may have a configuration that, for example as shown in FIG. 22, a ferromagnetic film 131, an intermediate film 132, and a ferromagnetic film 133 are formed in order from the side of the intermediate layer 122 and the ferromagnetic films 131 and 133 are exchange-coupled. Such a spin valve structure is formed by a method of sputtering, vacuum deposition, or the like.
The materials and the like of the pinned layer and the free layer in the GMR element used for a thin film magnetic head are disclosed in, for example, U.S. Pat. No. 5,549,978. The material of the intermediate layer sandwiched by the pinned layer and the free layer is generally, for example, copper (Cu). A GMR element capable of using so-called tunnel effect obtained by making a very thin intermediate layer (tunnel barrier layer) of an insulating material such as aluminum oxide (Al2O3) in place of copper was also developed.
In the GMR element used for a thin film magnetic head, the magnetization direction of the free layer freely changes according to a signal magnetic field generated from a magnetic recording medium. At the time of reading magnetic information recorded on a magnetic recording medium, for example, read current is passed along a stacked-body in-plane direction to the GMR element. At this time, the read current displays an electric resistance value which varies according to the state of the magnetization direction of the free layer. Consequently, a change in the signal magnetic field generated from the recording medium is detected as a change in electric resistance.
This phenomenon will be described in more detail by referring to FIGS. 23A and 23B. FIGS. 23A and 23B show the relation between the magnetization directions of the pinned layer 121 and free layer 123 and the electric resistance of the read current in the spin valve structure. The magnetization direction of the pinned layer 121 is indicated by reference numeral J121 and that of the free layer 123 is indicated by reference numeral J123. FIG. 23A shows a state where the magnetization directions of the pinned layer 121 and the free layer 123 are parallel to each other, and FIG. 23B shows a state where the magnetization directions of the pinned layer 121 and the free layer 123 are anti-parallel to each other. In FIGS. 23A and 23B, in the case of passing read current in the stacked-body in-plane direction, it is estimated that the read current flows mainly in the intermediate layer 122 having high electric conductivity. Electrons “e” flowing in the intermediate layer 122 are subjected to either scattering (which contributes to increase in electric resistance) or mirror-reflection (which does not contribute to increase in electric resistance) in an interface K123 with the free layer 123 and an interface K121 with the pinned layer 121. In the case where the magnetization directions J121 and J123 are parallel to each other as shown in FIG. 23A, the electrons “e” having spins Se parallel to the directions are not so scattered by the interfaces K121 and K123 and relatively low electric resistance is displayed. However, in the case where the magnetization directions J121 and J123 are anti-parallel to each other as shown in FIG. 23B, the electrons “e” are easily scattered by the interface K121 or K123 and relatively high electric resistance is displayed. FIG. 23B shows a state where the electron “e” having the spin Se to the right side of the drawing sheet is scattered by the interface K123 with the free layer 123. As described above, in the GMR element having the spin valve structure, electric resistance of the read current changes according to the angle of the magnetization direction J123 with respect to the magnetization direction J121. Since the magnetization direction J123 is determined by the external magnetic field, as a result, a change in the signal magnetic field from a recording medium can be detected as a resistance change in the read current.
Usually, a GMR element having the spin valve structure is constructed so that the magnetization direction of the free film (free layer) and that of the magnetization pinned film (pinned layer) are orthogonal to each other when an external magnetic field is not applied (H=0). The direction of the easy axis of magnetization of the free layer is set to be the same as the magnetization direction of the pinned layer. The GMR element with such a configuration is disposed so that the magnetization direction of the pinned layer is parallel to the direction of application of the external magnetic field. In such a manner, the center point of an operation range of the magnetization direction in the free layer can be set to the state where no external magnetic field is applied (H=0). That is, the state where the external magnetic field is zero can be set to the center of an amplitude of electric resistance which can be changed by a change in the external magnetic field. Consequently, it is unnecessary to apply a bias magnetic field to the GMR element.
The above will be concretely described with reference to FIGS. 24A to 24C and FIG. 25. FIGS. 24A to 24C show a state where magnetic information on a recording medium is read by a thin film magnetic head on which the GMR element is mounted in a general hard disk drive. As shown in FIG. 24A, the GMR element 120 is disposed close to the recording face 110 of a recording medium so that the magnetization direction J121 of the pinned layer 121 is the +Y direction along the direction (Y axis direction) orthogonal to the recording face 110 of the recording medium and the magnetization direction J123 of the free layer 123 is the +X direction along the direction (X axis direction) of the track width of the recording medium. It is assumed that there is no influence of the signal magnetic field from the recording medium at this time point. When the hard disk drive is driven and, for example as shown in FIG. 24B, a magnetic field in a signal magnetic filed direction J110 in the −Y direction is generated from a recording medium, the magnetization direction J123 becomes the −Y direction which is the opposite to the magnetization direction J121. Therefore, the resistance value of the read current increases as described with reference to FIGS. 23A and 23B. On the other hand, for example, in the case where a signal magnetic field in the signal magnetic field direction J110 from the recording medium is in the +Y direction as shown in FIG. 24C, the magnetization direction J123 becomes the +Y direction which is the same as the magnetization direction J121. Therefore, the resistance value of the read current decreases. By making, for example, the state of FIG. 24B associated with “0” and making the state of FIG. 24C associated with “1” by using the resistance change, the signal magnetic field can be detected as binary information. As obvious from FIGS. 24A to 24C, the center of the amplitude of the magnetization direction J123 is the state of FIG. 24A (H=0). FIG. 25 shows the relation between the external magnetic field (signal magnetic field) H and electric resistance R in the GMR element 120. In FIG. 25, the external magnetic field in the −Y direction in FIGS. 24A to 24C is set as H>0 and that in the +Y direction is set as H<0. As shown in FIG. 25, as the intensity of the signal magnetic field in the −Y direction increases, the electric resistance R increases and is saturated in the end. As the intensity of the signal magnetic filed in the +Y direction increases, the electric resistance R decreases and is saturated in the end. In such a manner, the electric resistance R changes around the state where the external magnetic field H is zero as a center. Therefore, the GMR element in which the magnetization direction of the free layer and that of the pinned layer are orthogonal to each other at the zero magnetic field does not have to have bias applying means in particular, so that it is generally applied to read magnetic information recorded on a hard disk, a flexible disk, a magnetic tape, or the like. Orthogonalization of the magnetization directions is realized by performing, mainly, a regularization heat treatment process which determines the magnetization direction of the pinned layer and an orthogonalization heat treatment process which follows the regularization heat treatment process and determines the magnetization direction of the free layer.
FIGS. 26A to 26C show the outline of a process of forming the stacked body 120 in which the magnetization direction J121 of the pinned layer 121 and the magnetization direction J123 of the free layer 123 are orthogonal to each other. Concretely, first, while applying a magnetic field H101 in the +X direction for example, the free layer 123 is formed by sputtering or the like and the direction AE123 of the easy axis of magnetization is pinned (refer to FIG. 26A) and, after that, the intermediate layer 122 and the pinned layer 121 are sequentially formed. As shown in FIG. 26B, while applying a magnetic field H102 in the direction (for example, +Y direction) orthogonal to the magnetic field H101, annealing process is performed at a predetermined temperature (regularization heat treatment process). By the process, the magnetization directions J121 and J123 are aligned in the direction of the magnetic field H102. Further, as shown in FIG. 26C, while applying a magnetic field H103 of relatively low intensity in the direction (+X direction) orthogonal to the magnetic field H102, annealing process is performed at a rather low temperature (orthogonalization heat treatment process). By the processes, while the magnetization direction J121 is pinned, only the magnetization direction J123 is directed again to the +X direction. As a result, the stacked body 120 in which the magnetization directions J121 and J123 are orthogonal to each other is completed.
The GMR element having the spin valve structure subjected to the orthogonalization heat process is effective to obtain a high dynamic range as well as high output and is suitable for reproducing a magnetization inverted signal which is digitally recorded. Before such a GMR element is used, an AMR element using anisotropic magneto-resistive (AMR) effect was generally used as means for reproducing a digital recording signal. Hitherto, the AMR element is used as means for reproducing not only a digital signal but also an analog signal (refer to, for example, Translated National Publication of Paten Application No. Hei 9-508214).
Application of the GMR element as means for reproducing an analog signal as well as the foreign AMR element has been being examined. However, when the free layer 123 in the GMR element subjected to the orthogonalization heat treatment is observed from a microscopic viewpoint, as shown in FIG. 27, it is found that spin directions 123S in magnetic domains 123D partitioned by magnetic walls 123W are various and are not aligned in a predetermined direction. Such variations in the spin direction 123S appear as a hysteresis characteristic in the relation between the external magnetic field H and the electric resistance R. FIG. 25 corresponds to an ideal state in which the spin directions in the magnetic domains in the free layer are perfectly aligned in one direction. In reality, however, the spin direction 123S varies, so that a hysteresis curve HC1 is drawn as shown in FIG. 28 and an offset value occurs at the zero magnetic field. The occurrence of the offset value appears as 1/f noise in a relatively low frequency band as shown in FIG. 29. The 1/f noise occurs at a frequency “f” or lower and becomes more conspicuous as the frequency “f” decreases. FIG. 29 shows a state where the influence of a 1/f noise component N2 on “noise voltage density” increases as compared with a white noise component N1 as the frequency “f” decreases. Increase in the 1/f noise is unpreferable since it is a big factor of deteriorating the reliability of the whole system.